This invention relates generally to computed tomography (CT) imaging apparatus; and more particularly, to the processing of scan data collected when the CT gantry is tilted.
In at least one known computed tomography (CT) imaging system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the “imaging plane”. The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a “view”. A “scan” of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from the scan into integers called “CT numbers” or
“Hounsfield units”, which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
Cone beam scanning is performed using a multi-dimensional detector array instead of a linear detector array as is used in a fan beam scan. In a cone beam helical scan, the x-ray source and the multi-dimensional detector array are rotated with a gantry within the imaging plane as the patient is moved in the z-axis synchronously with the rotation of the gantry. Such a system generates a multi-dimensional helix of projection data. In a cone beam step-and-shoot scan, the table is held stationary while the x-ray source and multi-dimensional detector array are rotated about the z-axis and a slice of data is acquired. The table is then moved to a next position along the z-axis to acquire the next scan slice. This process is repeated until the desired portion of the patient has been scanned.
In many clinical applications, the CT gantry is tilted to avoid direct exposure of the patient retina to the x-ray radiation or to ensure optimal spatial resolution. In the past, several algorithms have been proposed to correct the artifacts caused by the geometric change in the gantry tilt. These algorithms deal only with multi-slice configurations in the fan beam or cone beam reconstruction process. That is, the projection samples are filtered and backprojected in the same geometry as the data acquisition. Consequently, all the samples from a single view have a common intersection point which corresponds to the x-ray source location. It has been shown that such reconstruction process produces inferior noise characteristics due to the scaling factor in the backprojection step (the scaling factor is proportional to the inverse of the distance squared from the reconstructed pixel to the x-ray source).
Thus, a system and method are desired to improve the image noise homogeneity when the CT gantry is tilted that addresses the problems notes above and others previously experienced.